Apparatus for calibrating a mass air flow sensor using an oscillating air flow

ABSTRACT

A method and apparatus for calibrating a mass air flow sensor in which the sensing elements of the sensor are subjected to an oscillating mass air flow summed with an offset mass air flow to produce the summed mass air flow. The output waveform of the mass air flow in response to the summed mass air flow is displayed on a display having a desired waveform superimposed thereon. Selected resistors in the sensor&#39;s electronic circuit are then trimmed to conform the waveform output by the mass air flow sensor with the superimposed desired waveform.

This is a divisional of application Ser. No. 08/056,277 filed on May 3,1993, now U.S. Pat. No. 5,390,528.

TECHNICAL FIELD

The invention is related to the field of mass air flow sensors and, inparticular, to a method and apparatus for calibrating a mass air flowsensor using an oscillating mass air flow.

BACKGROUND ART

The calibration of the various transfer functions of mass air flowsensors of the type taught by Sato et al in U.S. Pat. No. 4,393,697 isconventionally conducted on separate calibration test stands. The massair flow sensor has a sensing head supporting sensing elements which aresensitive to a mass air flow rate. In current automotive applications,the sensing elements are normally entrained in an isolated or bypasschannel of the engine air intake system and measures the mass air flowrate of a portion of the total mass air flow. The mass air flow sensorgenerates an output signal corresponding to the mass air flow rate ofthe air in the isolated or bypass channel. The relationship of the valueof the sensor's output signal to the mass air flow rate is called atransfer function. This transfer function is normally calibrated inthree separate steps. First, the sensor is calibrated to produce aspecified signal in response to a preselected mass air flow rate. Next,the gain of the sensor is calibrated to generate a specified change inits output signal in response to a preselected change in the mass airflow. Finally, the response time of the sensor's output signal iscalibrated to have a predetermined response time in response to a stepchange in the mass air flow rate.

The calibration of these parameters, in production, are performed atseparate test stations while the sensor is mounted on a test stand inwhich the sensing elements are exposed to mass air flow rates controlledby sonic nozzles. The sonic nozzles accurately control the mass air flowrate required for the desired calibration. The calibration test standshave inlet geometries which condition the inlet air in an attempt toreduce turbulence of the mass air flow being sensed. However, turbulenceis almost always inherent in a system where air is drawn in by apressure drop or vacuum system where it is very difficult to achieveideal inlet geometry. The generation of turbulence through the air inletof these calibration stands result in unrepeatable flow fields and anincreased signal-to-noise ratio in the output signal generated by themass air flow sensor.

SUMMARY OF THE INVENTION

The invention is a method and apparatus for calibrating a mass air flowsensor. The method consists of mounting a mass air flow sensor on acalibration stand, generating an oscillating mass air flow andsubjecting the sensing elements of the mass air flow sensor to theoscillating mass air flow causing the mass air flow sensor to generatean output waveform having a frequency and amplitude indicative of theoscillating mass air flow. The method then displays the output waveformon a display having a waveform superimposed thereon. The calibrationprocedure then proceeds to laser trim a first resistance in the mass airflow sensor's electronic circuit to center the displayed output waveformwith the desired waveform, trimming a second resistance in the sensor'selectronic circuit to calibrate the amplitude of the displayed outputwaveform to be the same as the amplitude of the desired waveform andtrimming a third resistance to calibrate the response time to agree withthe response time of the desired waveform.

In the preferred embodiment, the method further includes generating anoffset mass air flow which is summed with the oscillating mass air flow.The offset mass air flow has a value selected such that the summed massair flow is unidirectional in the region of the sensing elements of themass air flow sensor. In the disclosed embodiment, the mass air flowsensor is attached to a sensor mount having an air flow conduit throughwhich the sum of the oscillating and offset mass air flow passes.

The object of the present invention is a method and apparatus for thecalibration of a mass air flow sensor Which produces a very repeatableair flow across the sensing elements of a mass air flow sensor therebyreducing the signal-to-noise ratio of the sensor during calibration.

On advantage of the method is that three separate calibrations of themass air flow sensor can be performed in one step on a single teststand.

Another advantage is that the oscillating air through the air flowconduit of the mass air flow sensor substantially reduces theturbulence, thereby reducing the signal-to-noise ratio of the sensor'soutput signal during calibration making the calibration more accurate.

Another advantage is that the production calibration time of the massair flow sensor has been reduced from approximately 2 minutes to 30seconds.

These and other advantages of the method and apparatus for calibrating amass air flow sensor using an oscillating air flow will become moreapparent from a reading of the specification in conjunction with thedrawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view showing the details of thecalibration apparatus;

FIG. 2 is a graph showing the waveforms of the generated oscillatingmass air flow and the output signal of the mass air flow sensor; and

FIG. 3 is an abbreviated circuit diagram of the mass air flow sensor'selectronic circuit.

DETAILED DESCRIPTION OF THE INVENTION

The details of the system for calibrating a mass air flow sensor usingan air flow that is oscillated between predetermined values are shown inFIG. 1. In the illustrated embodiment, a mass air flow sensor 10 of thetype having a hot element R_(H) and a cold element R_(C) is fixedlyattached to a sensor mount 12 having an air flow conduit 14. The sensinghead 16 of the mass air flow sensor 10 projects into the internalpassageway 18 of the air flow conduit 14 exposing the hot element R_(H)and the cold element R_(C) to the air flow therethrough.

The sensor mount 12 is mounted on a mounting fixture 20 defining a testchamber 22. One end of the test chamber 22 is covered by air filter 23and the opposite end is connected to a small diameter portion 24 of afunnel-shaped member 26. A resilient seal 28 is provided adjacent to thesmall diameter portion 24 of the funnel-shaped member which is sealinglyengaged by the outlet end 30 of the air flow conduit 14 when the sensormount 12 is attached to the mounting fixture 20. A resilient seal 28provided at the end of the small diameter portion 24 provides apneumatic seal between the outlet end 30 of the air flow conduit 14 andthe small diameter portion 24 of the funnel-shaped member 26.

The large diameter end 32 of the funnel-shaped member 26 is connected toan air flow oscillating mechanism 34. In a preferred embodiment, the airflow oscillating mechanism 34 consists of a cylinder 36 having adiameter substantially the same as the large diameter end 32 of thefunnel-shaped member 26, a piston 38 disposed in the cylinder, arotating crank mechanism 40 and a connecting rod 42 connecting therotating crank mechanism 40 to the piston 38. The rotating crankmechanism 40 is rotated by an electric motor 44. The connecting rod 42is connected to a crank pin 46 offset from the axis of rotation of thecrank mechanism 40 such that the piston 38 is reciprocated within thecylinder 36 in response to the rotation of the crank mechanism 40 by theelectric motor 44. In the preferred embodiment, the piston 38 isreciprocated at a frequency of 50±10 Hertz.

The reciprocation of the piston 38 in cylinder 40 produces anoscillating motion of the air within the funnel-shaped member 26 andwithin internal passageway 18 of the air flow conduit 14.

A pressure damping chamber 48 is provided adjacent to the small diameter24 of the funnel-shaped member 26. The annular pressure damping chamber48 preferably circumscribes the small diameter portion 24 and isconnected to the interior of the funnel-shaped member 26 by an annularport 50. A sonic nozzle 52 is connected between the damping chamber 48and a regulated vacuum source 54 illustrated as a vacuum pump 56 and avacuum tank 58. As is known in the art, sonic nozzles, such as sonicnozzle 52, are used to precisely control the mass air flow in apneumatic system. The sonic nozzle 52 is selected to provide apredetermined offset mass air flow through the air flow conduit 14 ofthe mass air flow sensor 10 which is greater than the mass air flowgenerated by the piston 38 when it is moving toward the small diameterportion 24 of the funnel-shaped member 26. This guarantees that the massair flow through the air flow conduit 14 will always be unidirectionaland flow in a direction from its inlet end 60 to its outlet end 30 asindicated by arrow 62.

As shown in FIG. 2, the offset mass air flow through the sonic nozzle 52offsets the oscillatory mass air flow produced by the reciprocation ofthe piston 38. The offset oscillating mass air flow produced by thesonic nozzle is depicted by the dash-dot line 64, while the outputsignal of the mass air flow sensor in response to the offset oscillatorymass air flow through the air flow conduit 14 is illustrated by solidsine wave curve 66. In FIG. 2, the oscillating mass air flow through airflow conduit 14 has its peak value when the piston 38 is moving awayfrom the air flow conduit 14 and has its minimum values when the piston38 is moving toward the air flow conduit 14. As previously discussed,the offset mass air flow through the sonic nozzle 52 is selected so thatthe sum of the offset and oscillating mass air flow through the air flowconduit is always unidirectional and flows in the direction of arrow 62.

To prevent the oscillatory mass air flow generated by the reciprocationof piston 38 from adversely affecting the mass air flow through thesonic nozzle 52, the volume of the damping chamber 48 is selected to besignificantly larger than the air volume displaced by the reciprocatingpiston 38. Preferably, the volume of the annular damping chamber 48 isfive (5) or more times the volume displaced by the piston 38 so that theoscillating air flow produced by the piston 38 will not significantlychange the pressure within the annular damping chamber 48 nor the massair flow through the sonic nozzle 52. Further, isolation of the pressureof the air inside the annular damping chamber 48 may be achieved bycontrolling the cross-sectional area of the annular port 50 such thatthe pressure inside the damping chamber 48, due to the offsetoscillating mass air flow through the sonic nozzle 52 is slightly lessthan atmospheric pressure. This will further isolate the pressure insidethe damping chamber 48 from pressure fluctuations caused by theoscillations of the air flow inside the funnel 26.

The calibration of the mass air flow sensor 10 using the oscillating airflow through the air flow conduit 14 of the sensor mount 12 shall bediscussed relative to the waveforms shown in FIG. 2 and the electroniccircuit of the mass air flow sensor 10 shown in FIG. 3. The circuitshown in FIG. 3 is an abbreviated circuit diagram showing only theessential elements of the circuit involved in the calibration of themass air flow sensor 10.

Referring to the circuit diagram shown in FIG. 3, the sensing head 16 ofthe mass air flow sensor 10 has a hot or heated element R_(H) and a coldelement R_(c) which are disposed in the internal passageway 18 of theair flow conduit 14. The heated element R_(H) and the cold element R_(c)have a positive thermal coefficient of resistance α such that theirresistances increase with temperature.

The heated element R_(H) is connected in series with a resistance R₁between the output of operational amplifier 70 and ground. A parallelvoltage divider network consisting of serially connected resistances R₂,R₃, R₅ and R₆ is also connected between the output of operationalamplifier 70 and ground in parallel with the heated element R_(H) andresistance R₁. The junction between resistances R₂ and R₃ is connectedto a negative input of operational amplifier 70, while the junctionbetween the heated element R_(H) and R₁ is connected to the positiveinputs of operational amplifiers 72 and 74 through a resistance R₉. Theoutput of operational amplifier 72 is connected directly to the positiveinput of operational amplifier 70 and to its own negative input throughcold element R_(C) and a resistance R₇. The junction between coldelement R_(c) and resistance R₇ is connected to the junction betweenresistances R₃ and R₅ and to ground through a resistance R₈.

A regulated voltage, designated V_(cc) is connected to the positiveinputs to operational amplifiers 72 and 74 through a resistance R₁₁, tothe junction between resistance R₅ and R₆ by a resistance R₄, and to thejunction between serially connected resistances R₁₂ and R₁₃ connectedbetween the negative input to operational amplifier 74 and ground by aresistance R₁₄. The output of operational amplifier 74 is connected toan output terminal 76 and to its negative input by a resistance R₁₀.Resistance R₁₀ is a feedback resistor controlling the gain of operationamplifier 74. The output of operational amplifier 74 appearing on outputterminal 76 is the output signal of the mass air flow sensor 10indicative of the mass air flow rate sensed by the heated element R_(H)and the cold element R_(C).

The circuit shown in FIG. 3 is basically a balanced bridge circuit inwhich the output of operational amplifier 70 is controlled by the outputof operational amplifier 72 to maintain the bridge balanced. The heatedelement R_(H), with no mass air flow through the air flow conduit 14, isheated to a predetermined temperature. The heated element R_(H) will becooled by an air flow through the air flow conduit 14 which, in turn,decreases its resistance. A decrease in the resistance of the heatedelement, in turn, increases the potential at the positive input ofoperational amplifier 72'. The increase of the potential at the positiveinput of operational amplifier 72 increases the potential at thepositive input of operational amplifier 70 which, in turn, increasespotential across heated element R_(H) and resistance R₁. This processproceeds until the bridge becomes balanced. The potential at thejunction between the heated element R_(H) and R₁ increases with theincreased potential of the output of operational amplifier 70 and isindicative of the mass air flow across the heated element R_(H).

The value of the resistance R₆ controls an offset value of the outputsignal generated by the operational amplifier 74 so that its outputsignal will have a predetermined or selected value for a specified massair flow rate through the air flow conduit 14 which is controlled by thesonic nozzle 52.

The resistance R₄ controls the response time of the mass air flow sensor10 to a change in the mass air flow rate produced by the reciprocatingpiston, and the feedback resistance R₁₀ controls the gain of operationalamplifier 74.

During the test procedure, the air flow through the air flow conduit isregulated to have a predetermined average mass air flow rate determinedby the sonic valve 52 while the reciprocating piston 38 oscillates themass air flow rate through the air flow conduit as shown by curve 66 inFIG. 2. The output of operational amplifier 74 is then displayed on acathode ray tube 78 having the shape of the desired waveform 80superimposed thereon. Resistance R₆ is first laser-trimmed to center thewaveform 82 output by the mass air flow sensor 10 with the desiredwaveform 80. Resistance R₁₀ is then laser-trimmed to calibrate theamplitude of the waveform 82 output by the mass air flow sensor 10 untilits amplitude is substantially equal to the amplitude of the desiredwaveform 80. Finally, resistance R₄ is laser-trimmed until thecross-over points of the output signal 82 of the mass air flow sensorwith respect to the offset potential 64 corresponds to the cross-overpoints of the desired waveform 80. A final laser-trimming of resistancesR₆ and R₁₀ may be required to complete the calibration of the mass airflow sensor to conform to the desired waveform 80. In FIG. 3, theillustrated output waveform 82 represents the waveform generated by themass air flow sensor 10 prior to calibration. After calibration, thewaveform 82 is substantially identical to the desired waveform 80.

By using an oscillating mass air flow through the air flow conduit 14 ofthe sensor mount 12, the output signal of the mass air flow sensor maybe calibrated with reference to three different operational values,namely, (1) the calibration of the mass air flow sensor's output signalto have a potential having a specified value for a predetermined massair flow, (2) the calibration of the gain of the output signal of themass air flow sensor so that the potential of the output signal willchange by a specified value in response to a predetermined change in themass air flow being detected, and (3) the calibration of the responsetime of the mass air flow sensor's output signal to be within specifiedlimits.

Having disclosed the method and apparatus for calibrating the output ofa mass air flow sensor using an oscillating mass air flow, it isrecognized that those skilled in the art may make certain changes andimprovements thereto with the scope of the invention as set forth in theappended claims.

We claim:
 1. An apparatus for calibrating a mass air flow sensor havinga sensing head projecting therefrom, said apparatus comprising:amounting fixture on which said mass air sensor is mounted, said mountingfixture defining an internal cavity receiving said sensing head, saidinternal cavity having an entrance port and an export port; meansconnected to said exit port for generating an oscillating mass flow;means for conducting said oscillating mass air flow to said sensing headof said mass air flow sensor received in said internal cavity meansconnected to said exit port for generating an offset mass air flow; andmeans for communicating said offset mass air flow to said sensing headof said mass air flow sensor received in said internal cavity togenerate an offset oscillating mass air flow.
 2. The apparatus of claim1 further including means for displaying an output signal generated bysaid mass air flow sensor in response to said offset oscillating massair flow.
 3. The apparatus of claim 1 wherein said means for generatingan oscillating mass air flow comprises:a cylinder connected to one endof said means for conducting; a piston disposed in said cylinder; andmeans for reciprocating said piston.
 4. The apparatus of claim 3 whereinsaid means for conducting said oscillating mass air flow to said sensinghead comprises a funnel-shaped member having a large diameter endconnected to said cylinder and a small diameter end connected to saidexit port.
 5. The apparatus of claim 3 wherein the means for generatingan offset mass air flow comprises:a sonic valve having an inlet endconnected to said funnel-shaped member and an outlet end; and a vacuumsource connected to said outlet end of said sonic valve.
 6. Theapparatus of claim 5 wherein said sonic valve is selected such that thesum of said offset mass air flow and said oscillating mass air flowproduces a unidirectional air flow through said air flow conduit of saidsensor mount.
 7. The apparatus of claim 3 further comprising:a pressuredamping chamber circumscribing said small diameter end of saidfunnel-shaped member; and an annular port provided through saidfunnel-shaped member connecting said pressure damping chamber to saidfunnel-shaped member.
 8. The apparatus of claim 4 wherein said mass airflow sensor includes a sensor mount having an air flow conduit disposedin said internal cavity when said mass air flow sensor is mounted onsaid mounting fixture, said air flow conduit having an outlet endaxially aligned with said exit port of said mounting fixture and whereinsaid mounting fixture further comprises a resilient annular sealsealingly connecting said outlet end of said air flow conduit to saidsmall diameter end of said funnel-shaped member.
 9. An apparatus forcalibrating a mass air flow sensor mounted on a sensor mount having anair flow conduit, said mass air flow sensor having a sensing headdisposed in said air flow conduit, said apparatus comprising:a mountingfixture on which said mass air flow sensor is mounted; means forgenerating an oscillating mass air flow oscillating at a predeterminedfrequency; a member connected between said means for generating anoscillating mass air flow and said air flow conduit to conduct saidoscillating mass air flow to said air flow conduit; means for generatingan offset mass air flow added to said oscillating mass air flow togenerate an offset oscillating mass air flow, said offset oscillatingmass air flow maintaining a unidirectional mass air flow through saidair flow conduit; a visual display for displaying an output signalgenerated by said mass air flow sensor in response to said offsetoscillating mass air flow; a desired waveform superimposed on saidvisual display to which said output signal generated by said mass airflow sensor is calibrated; and means for trimming selected resistancesin said mass air flow sensor to calibrate said output signal generatedby said mass air flow sensor to agree with said desired waveform. 10.The apparatus of claim 9 wherein said means for generating anoscillating mass air flow comprises:a cylinder connected to said member;a piston disposed in said cylinder; and means for reciprocating saidpiston in said cylinder at said predetermined frequency.
 11. Theapparatus of claim 10 wherein said means for generating an offset massair flow comprises:a damping chamber connected to said member; a portconnecting said damping chamber to the interior of said member; a sonicnozzle having an inlet end connected to said damping chamber and anoutlet end; and a vacuum source connected to said outlet end of saidsonic nozzle.